Corona is a luminous discharge surrounding high voltage conductors caused by ionization of the surrounding air. Corona occurs when the electrical fields generated by the high voltage conductors are sufficiently high to break down or ionize the air, and is always present to some degree in high voltage air-cooled ac motors and generators. Generally speaking, it is not practical to manufacture large air-cooled high voltage ac motors and generators to operate free of corona activity since doing so would adversely affect the size and efficiency of these machines.
Corona often causes oxidation of organic materials, resulting in discoloration and dusting. In air-cooled machines, the end winding surfaces on the coils and the structures that serve to separate and support the coils, including coil-to-coil space blocks, binding bands, radial rings and bar ties, are formed of organic insulating materials, and are therefore susceptible to deterioration by corona activity. The entire end winding assembly is usually coated with an organic finish paint that is also susceptible to deterioration by corona activity. The paint is usually applied by spraying to cover the end winding assembly and its connection rings and stator core to protect the winding and provide uniform surface coloring for ease of inspection. Paints currently in use on high voltage air-cooled generators and motors include water-reducible phenolic alkyd paints, acrylic waterborne paints, alkyd waterborne paints, acrylic-epoxy waterborne paints, two-component epoxy waterborne paints, and two-component epoxy solvent-borne paints that usually contain a solution of a high molecular weight bisphenol A--diglycidyl ether epoxy resin in a solvent, which is mixed with an amine-type curing agent immediately prior to being sprayed on the end winding assembly. While these coatings have performed well over the years, they are prone to discolor and form powdery deposits that cover the surfaces of stator bars and the support components of end windings.
Various factors affect corona levels and the associated surface discoloration and dusting of polymeric organic materials of end windings. Corona activity between adjacent coils in end windings is affected by coil-to-coil clearances, coil-to-coil voltages, contamination in the cooling gas (e.g., air), and the velocity of the cooling gas through the spaces in the end winding. Furthermore, coil-to-coil blocking and other support components in end windings can affect the surface voltage gradients along a coil, and therefore will also influence corona activity. Corona activity is usually the most severe at locations near coil-to-coil space blocks between phases, and at the connection ring assembly and support structure.
While discoloration and dusting can occur in the end windings of most air-cooled machines after long-term operation, deterioration of this type generally does not affect the service life of the motor or generator. Usually only the paint protecting the end winding is affected. In more severe circumstances, prolonged corona activity can penetrate the paint and begin to attack the ground insulation, but generally will not penetrate through the outermost layer of mica of the multiple layers of mica tape used as ground insulation. However, the effect of the corona activity is easily visible in many machines, and creates maintenance concerns that must be addressed.